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Synthesis of Nanocrystalline N-Doped TiO2 and Its Application on High Efficiency of Dye-Sensitized Solar Cells

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*Corresponding author.

E-mail address: irienuny@yahoo.com Ubon Ratchathani University

http://scjubu.sci.ubu.ac.th

Research Article

Synthesis of Nanocrystalline N-Doped TiO

2

and Its Application

on High Efficiency of Dye-Sensitized Solar Cells

C. Kusumawardani

1

*, K. Indriana

2

, Narsito

2

1

Chemistry Department, Yogyakarta State University, Yogyakarta 55281, Indonesia.

2

Chemistry Department, Gadjah Mada University, Yogyakarta 55283, Indonesia. Received 22/12/09; Accepted 12/03/10

1. Introduction

Highly energy demand of our world onto fossil fuels has posed several drawbacks since fossil fuels are nonrenewable, causing many environmental problems and are likely not to continue to remain abundant for the next generations. Therefore, the search for alternative renewable energy technologies is of crucial importance for the sustenance and development of modern society [1]. One of the possible solutions to the energy challenge

is to make efficient use of solar energy, which is abundant, long lasting and clean. Solar cells are being the most area of interest in solar energy utilization because it can directly convert the solar energy to electric-ity.

A new type of solar cells developed by O’Regan and Grätzel in 1991, dye-sensitized solar cells (DSSCs) have been attracting much attention over last decade as potential low-cost alternative to commercial solar cells based on silicon due to their ease of fabric-ation and high photo-conversion efficiencies [2-5]. Despite low-cost, their system had a

Abstract

Nanocrystalline of Nitrogen-doped TiO2 (N-doped TiO2) has been synthesized through the hydrolysis of N-substituted titanium isopropoxide precursors. The XRD result showed that the structure of annealed N-doped TiO2 was anatase. The pore properties were investigated from nitrogen gas sorption analyzer, showing mesoporous structure of its N-doped TiO2 with a high specific surface area (167 m2/g) and a sharp pore radius distribution. The substitution of oxygen sites with nitrogen atoms in the titania structure was confirmed by X-ray photoemission spectroscopy (XPS) with 3.6% nitrogen contain. A clear decrease in the band gap on N-doped TiO2 (compared to Degussa P25) is deduced from the optical absorption spectroscopy results. Application of synthesized nanocrystalline N-doped TiO2 on dye-sensitized solar cells resulted 4.8% overall conversion efficiency, which was higher than the Degussa P25.

Keywords: N-doped TiO2, Hydrolysis, Dye-sensitized solar cells.

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light-to-electricity conversion efficiency of around 12%, a level that is not easily obtained since the first concept firstly noticed in early 1970 [6]. It is necessary to further improve the energy conversion efficiency in order to commercialize DSSC successfully. Many methods for improving the conversion efficiency of the DSSC have been attempted [7-10]. Some consider-able efforts have been devoted to find the most efficient dyes to increase the efficiency but effort to modify semi-conductor TiO2 as the most efficient support for the dye have been overlooked. Nanoparticles TiO2 and thin films TiO2 are easily produced, inexpensive and shown good stability under illumination in most environ-ments [11-13]. However, the intrinsic wide band-gap nature (around 3.2 eV) of TiO2 impairs TiO2 from playing such an important role, because it only allows TiO2 to adsorb UV light, which accounts for merely 5 % of the incoming solar energy on the earth’s surface [1]. To improve the photoactivity of TiO2, it is desirable to red-shift the absorption onset to also include the less energetic but more intense visible part of the solar spectrum [14]. Traditionally, this has been achieved by anchoring organic dyes as sensitizers, which are usually Ru(II) com-plexes, to harvest the visible light [2,9]. Although this method broadens the range of the visible light response effectively, the problem appear with organic dyes that they can detach from the surface when employed in aqueous solution and the long term stability of many dyes can be questioned. The use of pure TiO2 semiconductors, which there is some oxygen deficiency in the crystal structure can create electron-hole pairs [15-18] and that the oxidizing holes can either react with the dye and destroy it and/or is scavenged by iodide ions [20], may lead to shorten the lifetime of the dye-sensitized solar cells. Therefore, there is a need to increase the DSSC efficiency and stability by other approaches. To solve these problems, we introduce nitrogen-doped TiO2 into the DSSC system to enhance the efficiency due to the replacement of pure TiO2 by visible light active nitrogen-doped TiO2.

Nitrogen-doped TiO2 have been produced through various techniques, such as hydro-lytic process [15,19], mechanochemical tech-nique [21], reactive DC magnetron sputtering [22,23], solvothermal process [24], high temperature treatment of TiO2 under N atom-sphere [25,26] and sol gel method [1,12,14] whereby the approach involves the hydro-lysis of Ti precursor, in the presence of N-precursor such as ammonia and organic amines. Among these methods, the N-doped TiO2 nanoparticles are mainly obtained through the last two methods, but more commonly through sol gel approach because it affords simplicity in controlling the nitrogen doping level and particle size by simple variations in experimental condition, such as hydrolysis rate, solution pH and solvent systems. Burda et al. added triethyl-amine to the colloidal nanoparticle solution and heated the samples to obtain ~4% nitrogen-doped TiO2 with an aver-age grain size of 6-10 nm, which could absorb well into the visible region up to 600 nm, but the samples had to be prepared under low pH condition and temperature as low as 2oC through the sol gel process [18]. In this article, we report a sol gel method for synthesizing of visible light active nitrogen-doped (N-nitrogen-doped) TiO2 and its application into the DSSC system.

2. Theory

Improvement of TiO2 photoactivity through nitrogen doping could be determined by light absorption shift to visible region. It is quantified by bandgap energy which can be calculated by the equation [2]

Eg=1239.8/λ (1) where Eg is the band gap (eV) and λ (nm) is the wavelength of the absorption edges in the spectrum.

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computerized control of Keithley instrument with a 450 W Xenon lamp, which was focusing to provide 1000 W/m2, equivalent to one sun at AM 1.5 at the surface of the cells. The instrument was calibrated using silicon diode. The spectral output of the lamp was matched in the region 350-800 nm with the aid of a Schott KG-5 sunlight filter so as to reduce the mismatch between the simulated and the true solar spectrum to less than 2%. The current-voltage characteristics of the cells were determined by biasing the cells externally and measuring the generated photocurrents. The overall photo-conversion efficiency  is calculated from the integral photocurrent density (Isc), the open circuit

photocurrent (Voc), the fill factor of the cell

(ff), and the intensity of incident light (Is)

using the formula

I

sc

V

oc

ff

/

I

s (2)

Is = 1000 W/m2 at air mass (AM) 1.5 or under

full sunlight. Fill factor (ff) is given by

sc oc

pp pp

I

V

I

V

ff

(3)

where Vpp is a maximum voltage and Ipp is a

maximum current.

3. Materials and Methods

Materials. Titanium Tetra Iso-propoxide,

Ti(OPr)4, 97% and acetylacetone were purchased from Aldrich. Dodecylamine 98 % and TritonX-100 were purchased from Fluka. Ethanol absolute and CH3COOH were obtained from Merck. All materials were used as received. Di-tetrabutylammonium cis-di(isothio-cyanato)bis (2,2’-bipyridyl-4,4’ -dicarboxylato) Ru(II) (N719 dye), electro-lyte EL-HSE, TEC 15 electrode glass plate and Pt-coated counter electrode are comm-ercial products of Dyesol (Australia).

Synthesis of N-doped TiO2. For synthesis of

the N-doped TiO2 powders, a mixture of 3

mL of Ti(OPr)4, 10 mL dodecylamine, and 80 mL of ethanol absolute solution was refluxed for 4 hours at 70 oC to provide a clear solution. This precursor solution was then cooled to room temperature and 5 mL of CH3COOH was added to neutralize the excess of dodecylamine. Hydrolysis process was then achieved by adding 20 mL of distilled water dropwise into the solution under vigorous stirring, the solution conti-nues stirred for 24 hours. The resulting yellowish precipitate was centrifuged and washed subsequently with distilled water and ethanol. Finally, the N-doped TiO2 were vacuum-dried for 12 hours. The surfactant was removed from the as-made N-doped TiO2 powders by calcined at a heating rate of 2 oC/min in air atmosphere for 4 hours at 450 o

C.

Characterization. The N-doped TiO2 powder was analyzed by thermogravimetric analysis (TGA) with a Q500TGA Instrument using a 10 oC/min ramp up to 800 oC. The structure of the powder was examined with X-Ray powder diffractometer (XRD, Shimadzu) with Cu K radiation ( = 0.15406 nm), X-ray photo-electron spectroscopy (XPS, PHI-5300) and N2 adsorption-desorption measurements at 77 K (NOVA Quanta-chrome). UV-visible diffuse reflectance spectra were obtained for N-doped TiO2 using a UV-visible spectrophotometer (UV-2550, Shimadzu).

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4. Results and Discussion

N-doped TiO2 can be prepared in several ways. In this research we synthesized N-doped TiO2 through sol gel method by hydrolysis of N-substituted titanium iso-propoxide precursors in alcohol solution. We found that complexation of organic amines on the Ti metal center creates highly efficient precursors N-doped TiO2 nanoparticles. Advantages of this route are simple method, high doping levels, controllable structure and high surface area at low cost.

The crystal structures of the synthesized N-doped TiO2 were studied by X-ray powder diffraction (XRD), as shown in Figure 1. The peaks in the spectra indicated that the crystal phase of prepared N-doped TiO2 was anatase and that no crystal phase of the rutile was observed after annealing at 450 oC. The crystallite sizes of N-doped TiO2 is deter-mined to be 8 from the half-width of the (101) peak using the Scherrer formula.

Figure 1. XRD Spectra of N-doped TiO2 after calcination at 450 oC.

To understand the loss of organic resi-dues on the surface of the N-doped TiO2 under calcinations, we used the thermo-gravimetric analysis (TGA) to study the weight loss of the N-doped TiO2. The TGA results of the N-doped TiO2 in Figure 2 show that there is less weight loss, about 20% from room temperature up to 220 oC, which is mostly due to the loss of physically adsorbed or embedded water and solvent on the surface of the N-doped TiO2. From 220 to 420 oC, there

is about 20% weight loss of The N-doped TiO2 sample. From 420 to 450 oC there is much slower (about 2 %) weight loss of the N-doped TiO2 sample in association with the crystallization of these nanoparticles. Above 450 °C, there is very little, about 3% weight loss of the N-doped TiO2 sample in association with the crystallization of these nanoparticles. Annealing temperature of 450 o

C was chosen based on TGA data that the surfactant will be released by calcinations above 400 oC, but there still nitrogen left in the TiO2.

Temperature (oC)

200 400 600 800

W

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Ti2p

Binding Energy (eV)

454 456 458 460 462 464 466 468

Int

surface adsorbed ammonia, with the BE located at 396.0 eV. In the Ti2p3/2 XPS spectra (Figure 3), the major titanium peak is centered at 459.0 eV. There is an intermed-iate peak centered at ca. 457.5 eV which correspond to Ti-N bound. The XPS element-al anelement-alysis was resulted 3.6% nitrogen doped on TiO2.

Figure 3. XPS spectra of N1s (up) and Ti2p (down) of N-doped TiO2 calcined at 450 oC. Figure 4 shows the typical nitrogen isotherm of N-doped TiO2 and its corresponding pore radius distribution. The adsorption desorption curve exhibits a type-IV isotherm curve with an H2 hysteresis loop according to IUPAC classification [30], which means the material has mesoporous structure. The N-doped TiO2 shows high BET surface areas 167 m2/g because of the mesoporous structure and the large amount of nanometer crystallites. The pore radius distribution of N-doped TiO2 shows a sharp pore radius peak at 5.2 nm.

Figure 4. Isotherm curve of N-doped TiO2(a) and its corresponding pore distribution (b). The optical absorbance and reflectance was used to study the capability to photo-sensitize the TiO2 nanoparticles. The absorbance shift of the N-doped TiO2 NPs can be observed from the reflectance spectra of undoped (Degussa P25) and N-doped TiO2 NPs in Figure 5. The yellowish N-doped TiO2 sphere powders show good absorbance of visible light. It can be seen from Figure 5 that the visible light absorption is high and extended up to 550 nm in the case of N-doped TiO2 calcined at 450 oC compared to that of pure TiO2 which could only absorb light in the UV range. It may be due to nitrogen species occupy some of the oxygen positions in the lattice. This also rules out the occupancy of N in any other positions such as interstitial sites, which should give rise to a mid gap band/level between valence and conduction bands.

392 394 396 398 400 402 404

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The band gap for N-doped TiO2 and Degussa P25 were calculated by Eq. (1) for the absorption edge in the visible region was found to be 2.25 eV and 3.09 eV, respect-ively. It is evident that the conduction band edge has changed by nitrogen doping.

Figure 5. DRUV Spectra of N-doped TiO2 compared to Degussa P25.

Figure 6. Current-voltage curves of the dye-sensitized cell based on the N-doped TiO2 and Degussa P25.

Figure 6 shows the current-voltage curves of the open cells based on the N-doped TiO2 and Degussa P25 photoelectrodes. A pronounced increase in the photo-current for the DSSC based on the nitrogen-doped titania was observed. The performance parameters of the DSSCs are summarized in Table 1. A high-energy conversion efficiency of 4.8% was achieved, which was higher than that of the P25 (4.5 %), respectively.

It assumed that visible light absorption of nitrogen-doped TiO2 supports intrinsically increases the efficiency value due to the photoresponse of N-doped TiO2 in the visible light region, which is also supported by the results reported by Lindgren et al. [22]. They have demonstrated that the photoinduced current due to the visible light activity of the best N-doped TiO2 electrode prepared by reactive DC magnetron sputtering can in-crease significantly by approximately 200 times over those of the undoped TiO2 electrodes. On the basis of these results, it can be expected that the optimization of the amount of nitrogen doping in titania nano-particles and electrode can further improve the performance of the DSSCs.

Figure 7. Absorption spectra of desorbed dye after leaching process.

It also should be pointed out that a higher Ruthenium dye uptake was observed on the N-doped TiO2 film, compared to that for P25. It has been known that a larger surface area of the N-doped TiO2 film can increase the amount of dye uptake and further lead to an increase in the conversion efficiency of the dye-sensitized solar cell. The adsorption of the dye on both materials was characterized by desorbing the dye into KOH (1:1 etanol-/H2O) and measuring their light absorption. The UV Visible absorption spectra (Figure 7) clearly showed higher dye absorption on the N-doped TiO2.

400 500 600 700 800

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Table 1. Performance parameters of DSSCs based on N-doped TiO2 and Degussa P25.

Character N-doped TiO2 DSSC Degussa P25 DSSC

Isc (mA/cm2) 12.86 13.44

Voc (V) 0.68 0.63

Ipp (mA/cm2) 9.48 10.14

Vpp(V) 0.51 0.44

Ff (%) 0.55 0.58

(%) 4.80 4.50

5. Conclusions

The N-doped TiO2 nanocrystalline materials were synthesized successfully by a novel sol gel method. Three binding energy peaks were observed at 396.0, 399.4 and 400.5 eV in the N 1s region of the XPS. The first signal (around 396 eV) was assigned to the molec-ularly adsorbed nitrogen species. Whereas two signals on higher energy binding were attributed to a chemically bound N- species and the O-Ti-N linkages within the crystal-line TiO2 lattice, respectively. A new absorpt-ion light was observed for the UV-vis spectrum of the nitrogen-doped TiO2 in the

visible light region. The action spectrum of the DSSC based on the N-doped TiO2 was in agreement with the corresponding optical spectrum. The high energy conversion effici-ency was achieved successfully for the DSSC based on the nanocrystalline N-doped TiO2 electrode.

Acknowledgements

Financial support from Indonesian Govern-ment by Directorate General of Higher Education through “Hibah Penelitian untuk Mahasiswa Doktor” project is gratefully acknowledged.

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Gambar

Figure 2. TGA graph of N-doped TiO2.
Figure 5. The yellowish N-doped TiO22 sphere powders show good absorbance of visible
Figure 5. DRUV Spectra of N-doped TiO2compared to Degussa P25.
Table 1. Performance parameters of DSSCs based on N-doped TiO2 and Degussa P25.

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